The invention relates to the field of turbomachines and an injection device for injecting a mixture of air and fuel into a combustion chamber of a turbomachine.
It is more specifically concerned with an injection device equipped with two fuel circuits and relates to a novel Venturi the shape of which improves the carburation.
In the description which follows, the terms “upstream” or “downstream” will be used to denote the positions of structural elements relative to one another in the axial direction, the point of reference taken being the direction in which the gases flow. Likewise, the terms “internal” or “radially internal” and “external” or “radially external” will be used to denote the positions of the structural elements relative to one another in the radial direction, taking the axis of rotation of the turbomachine as the reference.
A turbomachine comprises one or more compressor(s) delivering pressurized air to a combustion chamber in which the air is mixed with fuel and ignited in order to create hot combustion gases. These gases flow towards the downstream end of the chamber towards one or more turbine(s) used to convert the energy thus received in order to drive the compressor(s) and provide the work needed, for example, to power an aircraft.
Typically, a combustion chamber used in aeronautics comprises an internal wall and an external wall, these walls being joined together at their upstream end by a chamber end wall. The chamber end wall has, circumferentially spaced, a number of openings each housing an injection device that allows the mixture of air and fuel to be conveyed into the chamber. Each injection device particularly comprises a fuel injector, radial swirl vanes, a Venturi, a bowl and a deflector, all joined together, the chamber end wall being fixed to the deflector.
The combustion chamber is supplied with liquid fuel, mixed with the air from a compressor. The liquid fuel is conveyed as far as the chamber by the injectors in which the fuel is vaporized into fine droplets. This vaporization is initiated at the injector by nozzles and is continued in the Venturi and the bowl under the effect of the pressurized air from a compressor. This pressurized air passes, on the one hand, between the radial swirl vanes of the injection device in order to cause the fuel sprayed by the injector to swirl and, on the other hand, through the orifices formed in various parts of the injection device, such as the bowl.
There are several types of injector: injectors through which a single fuel supply circuit runs and those through which two fuel supply circuits run.
In the case of injectors that have two fuel circuits, a first circuit, termed the primary circuit or idle circuit, operates alone between turbomachine ignition and a first rotational speed of the turbomachine that corresponds to the idling speed. A second circuit known as the secondary circuit or full throttle circuit, then operates together with the primary circuit between the rotational speed corresponding to the idling speed and a second rotational speed that corresponds to full throttle, that is to say to the highest rotational speed of the turbomachine.
The fuel may be injected into the combustion chamber either directly or indirectly. It is said to be direct injection when the fuel does not strike any other part between leaving the injector and reaching the combustion chamber. It is said to be indirect injection when the fuel, having left the injector, strikes some other part before reaching the chamber. Generally, the part struck is the Venturi of the injection device.
As illustrated in document FR 2 753 779, the angle of the sheet of fuel from the primary circuit, which forms a cone of fuel, is generally defined in such a way that the fuel never strikes the Venturi and that injection in the primary circuit is therefore direct injection, whereas injection in the secondary circuit is indirect injection, the fuel striking the Venturi as it leaves the injector. An arrangement such as this makes it possible to get around the negative effects observed when the sheet of fuel of the primary circuit also strikes the Venturi. These negative effects are a prohibitive increase in the angle of the sheet of fuel leaving the Venturi and an increase in the droplet size. This causes unvaporized fuel to be sprayed on to the walls of the combustion chamber and leads to significant amounts of carbon monoxide and unburnt hydrocarbons being formed.
However, the fact that the injection in the primary circuit is direct injection means that when the primary circuit is operating alone, the Venturi is not struck with fuel and is therefore not cooled whereas when the primary and secondary circuits are operating, it is. Hence, the Venturi experiences significant variations in temperature which encourage the formation of coke.